Traditionally, electric power from a source to a load is transferred via wires, which obviously requires that the load device is connected to the source via wires. The desire to be able to move the load device leads to the solution of having connectors in the wires, such connectors making mechanical (Ohmic) contact. There are however examples where it is desirable to move the load device often, in which case the need to often connect/disconnect such connector becomes a burden. Practical examples include electric toothbrushes, telephones, and also lamps. To meet such desire, wireless power transfer has been developed. In such devices, connectors involve an inductive or a capacitive coupling. Typically, one of the coupling components is arranged within or attached to the housing of the load device itself. For instance, in the case of inductive coupling, the housing of the load device may contain a receiver coil, which for power transfer will be coupled to a transmission coil in a docking station, the two coupled coils basically constitute a transformer. In the case of capacitive coupling, the load device will include at least one receiver electrode which for power transfer will be coupled capacitively to a transmission electrode in a supply structure such as for instance a docking station. Such receiver electrode and transmission electrode are usually implemented as a plate, and when coupled they together define a capacitor.
FIG. 1 is a block diagram schematically illustrating a capacitive driving system 1, comprising a supply device 10 and a separate load device 20. The supply device 10 comprises two plate-shaped transmission electrodes 11, 12, which can be considered as output terminals. The supply device 10 further comprises a power generator 13 for generating AC power. A first output terminal 14 of the supply device 10 is connected to a first one 11 of the transmission electrodes, while a second output terminal 15 of the supply device 10 is connected to a second one 12 of the transmission electrodes. At least one inductor 16 is connected in series between the supply device 10 and the transmission electrodes 11, 12. The load device 20 comprises at least one load member 23 connected in series in between a first plate-shaped receiver electrode 21 and a second plate-shaped receiver electrode 22. The load member 23 is depicted as a resistor, and may ideally have ohmic characteristics. The transmission electrodes 11, 12 are located close to an outer surface of the supply device 10, and the receiver electrodes 21, 22 are located close to an outer surface of the load device 20. The disposition of the receiver electrodes 21, 22 matches the disposition of the transmission electrodes 11, 12, so that the load device 20 and the supply device 10 can be placed in close proximity of each other in an energy transfer position in which the first transmission electrode 11 together with the first receiver electrode 21 defines a first transfer capacitor 31 while simultaneously the second transmission electrode 12 together with the second receiver electrode 22 defines a second transfer capacitor 32.
The inductor 16 together with the capacitors 31 and 32 define a resonance circuit having a resonance frequency, and the power generator 13 is designed to generate an AC output signal at said resonance frequency, so that the circuit operates in resonance and power is efficiently transferred from the power generator 13 to the load member 23.
There are applications where the load device 20 is mounted to the supply device 10 once, and there are applications where the load device 20 is connected to and disconnected from the supply device 10 frequently. In any case, there exists a problem that the precise actual capacitance value of the transfer capacitors 31, 32 depends on the circumstances of the precise actual placement of the load device 20. A displacement of the load device 20 with respect to the supply device 10, or the accidental presence of pieces of dirt between the contacts, or, in cases where an additional dielectric contact liquid is applied, variations in the properties of the dielectric, will result in variation of the actual capacitance value of the transfer capacitors 31, 32, which in turn will result in variation of the actual resonance frequency and thus, since the power generator 13 is set to operate at the design resonance frequency, a variation in the power transferred to the load member 23.
Such variation is undesirable, with the level of inconvenience depending on the situation. In the case of a charger of an appliance, charging to the required level can take longer than expected, or the batteries are charged insufficiently and will be exhausted before this is expected. In case multiple mutually identical loads are driven in parallel, the loads receive different amounts of power. In the case of a lighting system having multiple lighting units driven in parallel, the respective lighting units produce mutually different output light levels, which is clearly visible to an observer.
For the developer and manufacturer of the driving system, such possible variations mean that there is uncertainty about the actual capacitance value of the transfer capacitors 31, 32, even if all components have been manufactured with great precision. If the manufacturer wishes to avoid the above-mentioned problems, adaptations to the power generator are needed.
US 2009/302690 A1 discloses a power transmission system that includes a power supplying apparatus and a power receiving apparatus. The power supplying apparatus comprises a power generator, a first resonance unit and power supplying electrodes. The first resonance unit comprises an induction component and/or a capacitance component and resonates the power signal, which resonated power signal is externally radiated by the power supplying electrodes. The power receiving apparatus comprises power receiving electrodes for receiving the radiated power signal and a second resonance unit that has an induction component and/or a capacitance component. The power supplying electrodes and the power receiving electrodes define transfer capacitors. To transmit power more efficiently by relaxing constraints on spatial relationships the power transmission system is provided with a control unit. The control unit controls the induction component and/or a capacitance component of the first or the second resonance unit based on the power value measured by the power measurement unit. This is a rather complicated way to compensate for spatial or placement deviations.